1. Field of the Invention
The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is concerned with a method of straightening an irradiated fuel assembly which has become bowed, the straightening being accomplished by adjusting the length of selected guide thimbles of the fuel assembly.
2. Description of the Prior Art
In most nuclear reactors, the reactor core is comprised of a large number elongated fuel assemblies. Conventional designs of these fuel assemblies include a multiplicity of fuel rods held in an organized array by grids spaced along the fuel assembly length. The grids are attached to a plurality of control rod guide thimbles. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the guide thimbles which extend above and below the opposite ends of the fuel rods.
For many years, bowing has been observed to occur in pressurized water nuclear reactor (PWR) fuel assemblies. As fuel assembly length and hold-down spring forces have gradually increased over the past years, the magnitude of fuel assembly bow has become more severe. A number of causes of fuel assembly bow have been identified; however, there is no practical design change to the fuel assembly which will eliminate its occurrence.
Fuel assembly bow impacts two areas of fuel assembly performance. First, bow in a fuel assembly makes it difficult to remove and replace it in the core without damage to adjacent fuel assemblies. Damage usually occurs to grids on the affected fuel assemblies. The second area concerns the nuclear performance of bowed assemblies. Nuclear calculations are based on a predetermined inter-assembly water gap. If the adjacent assemblies are bowed, a larger than anticipated water gap can exist around the outside fuel rods. This larger water gap results in a higher thermal neutron flux on the outer fuel rods than would be the case if the gap existing between the fuel assemblies was uniform. This leads to reduced thermal margins for the fuel rod cladding and can result in significant plant operational penalties.
For the above reasons, it is highly desirable to minimize fuel assembly bow. Consequently, a need exists for a fresh approach to correcting fuel assembly bow, which as presently understood is inevitable, in order to avoid its long-term deleterious effects on PWR fuel assembly performance.